As a waveguide slot-array antenna in which a slot is formed in a waveguide, there has been known one in which two slots called a slot pair are provided so that a slot length direction is perpendicular to an axis direction (line direction) of the waveguide. When an interval between the two slots is about a quarter of a guide wavelength, reflection from each slot in a slot array can be suppressed. In order to increase the amount of radiation from the slot, there has been proposed a method of offset-arranging (shifting and arranging) the two slots in the slot length direction (width direction of the waveguide broad-wall).
When the waveguide slot-array antenna is formed on a dielectric substrate, a broad-wall width of a waveguide which can propagate only a TE10 mode as a dominant mode is decreased by a wavelength shortening effect of a dielectric, and a resonant length of a slot and the broad-wall width of the waveguide are comparable to each other. Thus, an offset value is decreased when the slot pair is offset-arranged, the frequency bandwidth of the reflection characteristics from each slot is narrow, and there is a problem that the antenna efficiency is decreased.